How biomechatronic prosthetics are changing the face of disability

Hugh Herr claims semi-seriously to be "in the transportation business". There is a fairly significant twist, however, because, as he puts it: "I'm in the transportation business, but instead of big metal boxes with four wheels, I build body parts."

The body parts in question are bionic prosthetic limbs and Herr has an interest in them that extends far beyond the merely professional.

Why this should be is hinted at when Herr is asked what people need to become pre-eminent in their field in the way that he is. "Find a passion" is his answer and brief acquaintance with his biography quickly acquaints one with the source of his.

In 1982, when he was 17, Herr and a friend were climbing Mount Washington when the weather closed in and they were caught in a blizzard. After spending three nights in -29oC temperatures, both had suffered severe frostbite. For Herr this meant the amputation of both of his legs below the knee.

Where this story becomes truly remarkable, however, is that following months of surgery and rehabilitation, Herr was doing what doctors told him was unthinkable: climbing again using specialised prostheses of his own design. He used these to alter his height, avoid awkward body positions and grab hand and foot holds previously out of reach. Using these prostheses, his height was able to range from five to eight feet. As a result of using the prostheses, Herr went on to climb at a more advanced level than he had before the accident.

Today, Herr is an associate professor in MIT's Program in Media Arts and Sciences and in the Harvard-MIT Division of Health Sciences and Technology and head of the Biomechatronics research group at the MIT Media Lab. He is also the chief technology officer of Biom, a company specialising in bionic prosthetic limbs.

The key to this transformation, he feels, was the attitude he took to his situation after the loss of his legs. He says: "When my legs were amputated as a young man, I realised that – from the legs down – it was a blank palette. I could do whatever I wanted."

His positive attitude to his predicament has held Herr in excellent stead. He even describes himself as "lucky" to have lost both legs as "It's nice to be symmetrical and I have the added advantage that I can be as tall as I like". In fact, he sees this technology as utterly transformative, drawing a parallel between his prosthetics and the use of glasses in the way in which they effectively remove what was considered a disability and render the user a fully able, fully-functioning member of society. In this context, he says: "You cannot with a straight face label me as disabled, because with this great technology, I climb mountains, for God's sake!"

This most shocking thing, he felt, was discovering how primitive prosthetic technology was following his amputations. "Our current technology degrades us as humans rather than enhancing us," he says. "I was fitted with conventional technology in 1982. The foot/ankle was locked and the foot was made of foam and wood – fundamentally not distinct from prostheses that existed a hundred years before. And I was like 'Is this it?' In the age of computers and automobiles and spacecraft? That's when I really dedicated myself to designing something more successful... I didn't view my body as broken, I viewed the technology as broken. There is no such thing as a disabled person, there are only disabled technologies – there is only poor design."

The Biom ankle/foot prostheses Herr uses, by contrast, are 'intelligent' and responsive devices. Each contains three microprocessors and 12 sensors, which detect various forces, positions, speeds and accelerations. They deploy an Achilles-like tendon spring and a muscle-like motor system that catapults the person forward as they walk or run.

At the heel strike, a torsion spring under computer control responds. The stiffness of the spring depends entirely on the force of the impact. From this, the device employs torque control to lift the patient upwards and forwards into the swing step. These factors allow the Biom to adapt to speed and terrain automatically.

Says Herr: "The control system models the missing muscles, tendons and reflexes. And then we take sensory information from the bio-prosthesis and input that into bio physical model, which then tells the computers on board what the torque and impedences of the bionic limbs should be. So the bionic limb moves as if it's made of muscles and tendons and spinal reflexes. It moves as if it's made of flesh and bone.

According to Herr, however, the distinguishing feature of this prosthetic limb is that it is the first ever prosthetic limb not to be human-powered. "This applies tremendous mechanical energy into the human and augments their ability," he says.

"All other prostheses are essentially human-powered in the same way that a bicycle is human-powered and rely on residual muscle power to work. This device uses battery power. For the first time in history, this bionic limb normalises how fast a person can walk and how much energy they use to do so. It also greatly enhances stability: even on rough terrain, a person with both legs amputated can run up a hill."

Speaking of his own prostheses, Herr is effusive, saying: "When I walk, there's an exchange of information, an exchange of energy and an exchange of force between my biological self and my artificial self. It senses my biological postures and reflects its postures. It stores energy and catapults my biological self forward. My biological leg pushes on it, and it pushes back in a collaborative, seamless dance between flesh and machine."

All of these advances derive, claims Herr, from a concentrated effort to understand the human body better and to emulate it. He says: "Building a bionic limb that has the profound versatility of the human limb is incredibly hard. Right now when you open up your closet, you see lots of shoes. When I open up my closet, I see lots of legs. I have a leg for running, I have a leg for climbing – I have about eight pairs. It's a challenge to build all that capability into a single limb. The human leg is so adaptable and versatile.

"We've managed to capture the basics of the human ankle for level walking and running. But if you've got someone who wants to go hiking or dancing, for example – ie non-repetitive actions – then we have real problems."

The concentration on effective emulation of human limbs takes place at Herr's lab at MIT, where huge amounts of time and resource are spent on mapping, analysing and understanding human physiology. The fact that Herr is a user of these devices as well as their inventor and designer does, of course, give him a unique insight into their design, but in the case of his collaborators who have all their limbs, this has to be simulated.

"I'm very fortunate in that I'm a designer and I know physics and engineering," says Herr, "but also that I feel it physically. That generally means I'm the first one to know if there's a bug and also to know which line of code has to change. To be able to a feel it is a very powerful tool. Of course, the people in my lab have normal limbs, so for them we have all sorts of contraptions like struts that allow them to wear the limbs, too, so that they can feel it. It's a lot of fun."

Herr is very clear, however, that there remains a huge amount still to be achieved in this field, saying: "A dominant hurdle is our lack of understanding of how the human body works. It's essentially a better understanding of how the body works that led to what I'm wearing today... I just start running and it responds. There's no neural connection."

Clearly, then, Herr feels that the field of bionics has a long, long way to go yet. Just how far is hinted at when he says: "In this age of bionics, our limbs will no longer be separate lifeless mechanisms, but intimate extensions of our organic body," he says.

In fact, by outlining the current shortcomings of bionic technology, Herr points the way in terms of how he sees its development. The first problem he identifies are the power supplies, saying: "The battery on my limbs will go for about 3,000 walking steps, so if I'm going to take 10,000 steps in a day, I need extra batteries... The fact is that our motors and power supplies aren't yet as good – using several metrics – as human tissue, glucose and fat-driven power sources."

However, the more significant shortcoming, he believes, lies elsewhere. "The dominant problem with synthetics," he says, "is that our machines do not heal. We are not even close to being able to build a machine that lasts 80 years without repair. And yet a lot of us humans go 80 years without repair because the biological body is constantly monitoring and repairing."

By way of contrast, he argues: "$50m has been invested in this technology and the best we've been able to do is to last five years."

So what's coming next? The answer would appear to be neural connections that allow conscious control of and feedback from the limb. "The technology I'm wearing right now is very crude compared to where we'll be in a decade or two," says Herr. "There's an artificial intelligence in my current limbs, but there's no direct communication. But within ten years, limbs such as this will certainly be neutrally controlled. What will come first? The ability to think, send out the sensory command and then have that motor signal sent wirelessly to the limb.

"What will come after? The limb in action will stimulate the nervous system to give the person real perception of the synthetic limb. That's critical for balance, for instance. That feedback closes the loop between the human and the machine."

But Herr by no means sees bionics being limited to the limbs and other extremities. He finds the possibilities of bionic 'skin' particularly exciting,.

"The future," he says, "is that the retail outlet will have robots that will scan the foot and fit the shoe exactly to your body part. He says: "How can we interface virtually any structure to the human body? Bras? Shoes? We have no idea what we're doing. The shoe is one of the oldest devices in human culture and we still get blisters! My God! We have no idea what we're doing."

"The future," he says, "is that the retail outlet will have robots that will scan the foot and fit the shoe exactly to your body part. You won't get blisters. And that synthetic skin will be rich with sensors and electronics that will extract all sorts of physical data and respond accordingly – so relax and stiffen joints, for instance. It's coming. We ain't seen nothing yet!"

Indeed, far from being all about running faster and jumping higher (what he calls "all that Six Million Dollar Man stuff"), Herr sees the great challenge being how to understand, harness and even repair the brain. "I focus on how to get information out of the peripheral nerves. When we can do that, we can contemplate treating anxiety, schizophrenia, depression – all of those things".

The consequences of such an advance, he believes, could be immense. Not least because, as he points out, half the US population suffers some form of cognitive, emotional or physical disability.

As bionics become more sophisticated, the field of medicine will, Herr believes, have to consider a range of complex ethical issues, not the least of which will be whether to repair an existing limb or replace it with a bionic alternative. He says: "Eventually, we're going to get to a place where the bionic limb exceeds the capability of the biological limb. Today, people with severe problems with their limbs are asking doctors to amputate their limbs to improve their quality of life. If they turn on the TV and see me climbing or someone running very fast, they will ask themselves why they should be in that situation.

"When a person has a biological limb, but suffers from some pathology, they typically will go through surgery after surgery to retain some sort of capability. Now, however, there is debate in reconstructive surgery circles about the possibility of taking state-of-the-art limb transplantation and whether it should go biological or synthetic. In terms of patient outcomes and quality of life, there is a strong argument that bionic transplantation will make more sense."

Herr is in no doubt that this is where the technology is heading and his ambitions for it are huge. He says: "I envisage a world in which the technology is so advanced and the human/machine interaction so profound that we can rid the world of disability... and that's the goal: an end to disability."